US1811754A - Electrically heated salt bath furnace - Google Patents

Description

June 23, 1931. A. G. E. HULTGREN 1,811

EEECTRICALLY HEATED SALT BATH FURNACE Filed June 10, 1930 Patented June 23, 1931 UNITED STATES PATENT OFFICE MIGALLY mm SALT BATH FURNACE Application am June 10. mo. Serial 80. 400,231, and in Sweden February as, mo.

The present invention refers to electrically heated salt bath furnaces, to those emplofled for hard of highsteel too Two common esnso su furnaces I areshowninFig. land 2. Therefractory crucible 1 contains e molten salt 2. Heat is supplied by alternating current passing between the electrodes 3 through the salt. The tools are heated b immersion in the salt,

10 which is held at the esired temperature by regulating the current. Owing to convection currents, the temperature of the salt becomes very uniform. This, normally, applies to the temperature of the tool also, prol vided the time of immersion is suflicient. By measuring the temperature of the salt, the temperature of the tool, with the exception referred to below, is accurately determined. If the compositions of. the salt and the linlO ing are suitably chosen, oxidatlon of, or other chemical reaction with, the surface of the tool is practically prevented. This is not the case with certain other methods for heating of high-speed steel.

' In the arrangement shown in Fig. 1, the

electrodes are usually so connected to the current supply that the current passes between adjacent, not between opposite electrodes. In Fig. 2 three-phase current is used, the

current passing between each electrode and the two others. During the use of such furnaces, however, particularly when large objects were heated, signs of local or general overheating of the tool have often occurred, usually in the form of partial meltin of the steel. This has been so, in spite of t e temperature of the salt, as measured by pyrometer, having been correct and lower than the a melting point of the steel.

is that, owing to the higher conductivity of the steel as compared with that of the salt, and because of the distance between the tool and the corresponding electrodes having been I too small in comparison to the distance between the electrodes, a considerable part of the current has passed through the tool thus overheating it. The portions of the tool facing the electrodes are particularly liable to II overheating and melting phenomena. In

The explanation this way expensive tools have sometimes been destroyed.

In order to compensate for overheating from such cause, the salt may be kept at a correspondin ly lower temperature, but it is obvious that y this method uncertain and uneven tem ratures in the tool, sometimes too low an sometimes too high, will be attained.

According to the present invention, the current is passed practically entirely through the salt, not, or only a negligible part of it, through the immersed tool, even if the latter is re atively large, thus an undesirable and detrimental overheating o the steel being prevented. This is accomplished by a more suitable arrangement of the electrodes, preferably in connection with a more suitable shape of the latter and of the crucible.

Various designs of the invention are shown in Figs. 3 to 10, Figs. 3 to 5 adapted for one-p se current, a single circuit, Figs. 6 to 8 for one-phase current, two circuits, and Figs. 9 and 10 for three-phase current. As is seen, the electrodes are always arranged in pairs. In horizontal section, the inner faces of co-ordinated electrodes are parallel and comggratively close to each other, the distance in measured in the direction of the current. experiment, it has been determined that that distance should nowhere exceed 2 of the horizontal dimension of the salt bat in the same direction. Coordinated electrodes are preferably made to approach each other somewhat towards the bottom of the crucible. This feature, however, is not shown in the figures, which are schematic.

Experiments with the construction indicated in Figs. 3 to 10 have shown that there is no danger of overheating the immersed tool above' the temperature of the salt, if

the tool is kept outside of a zone surround ing the electrodes, the width of which is agproximatel equal to the distance between t e latter. though, according to the .invention only a small part of the salt is directly heated b the assing current, the temperature of t e who e bath becomes practlcally uniform, owing to the convection currents.

the objects to be heated, and the number of the objects to be heated simultaneously. After room has been provided for the purpose, one or more pairs of electrodes of appropriate size are added in the manner described above, and at a distance from the contour of said room equalto or somewhat exceeding the distance between two co-ordinated electrodes. The number of pairs of electrodes is determined by the type of current to be employed. Around the whole a figure is then circumscribed which forms the inner contour of the crucible.

As has been stated above, the inner faces of coordinated electrodes in horizontal sector, are preferably parallel, but deviations from this condition may be made without departing from the main principle of the invention.

In all the figures, straight lines have been drawn between co-ordinated electrodes in order to show schematically the paths of the current through the salt. On comparing Fig. 1 with Figs. 68 and Fig. 2 with Figs. 9-10, it is seen that, in the old designs of salt bath furnaces, the part of the salt available forheating of tools without danger of overheating b current passing through them, is considera 1y smaller than in the design ac cording to present invention.

In Figure 11 there is shown one form of my invention, wherein the electrodes of each pair approach each other towards the bottom of the furnace.

The present construction has been developed particularly for salt bath furnaces for hardening of high-speed steel but may also be applied to other salt bath furnaces. for heating of metal objects, in which heat is supplied by electric current passing through the salt between electrodes.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. Salt bath furnace for heating of tools and other metal objects, in which heat is supelectrodes, each pair of the said co-ordinated electrodes being so disposed that in an horizontal section the distance between a j acent faces is constant, or approximately constant, along their whole width.

3. Salt bath furnace for heating of tools and other metal objects, in which heat is supplied by electric current assing throu h the salt, having an even num er of electro es arranged in pairs in the crucible in such a manner that the space to be occupied by the immersed objects is not traversed by any straight current paths drawn between coordinated electrodes, the horizontal distance between the said co-ordina-ted electrodes in the direction of the current path nowhere exceeding half of the maximum internal width of the crucible in the same direction.

4. Salt bath furnace for heating of tools and other metal objects, in which heat is supplied by electric-current assing through the salt, having an even num er of electrodes arranged in pairs in the crucible in such a manner that the space to be occupied by the immersed objects is not traversed by any straight current paths drawn between coordinated electrodes, each pair of electrodes approaching each other towards the bottom of the furnace.

In witness whereof I have hereunto signed my name.

AXEL GUSTAF EMANUEL HULTGREN.

' plied by electric current passing throu h the salt, having an even number of electro es arranged in pairs in the crucible in such a manner that the space to be occupied by the immersed objects is not traversed by any straight current paths drawn between coordinated electrodes.

2. Salt bath furnace for heating of tools and other metal objects, in which heat is supplied by electric current assing throu h the salt, having an even num er of electro es arranged in pairs in the crucible in such a manner that the space to be occupied by the immersed objects is not traversed by any straight current paths drawn between co-ordinated III

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